Make imgdiag diff against app vs zygote heap

When both image pid and zygote pid are passed in, diff against the
zygote heap to show what fields caused private dirty pages.

Also fixed a bug where the zygote contents vector was reserved
instead of resized.

Bug: 63998303
Bug: 62554875
Test: test-art-target-imgdiag_test

Change-Id: Ifb801091e02e371657e48b14da709bb2b52dde51